🩹(all) clear the SonarCloud reliability finding and the lint debt

The SonarCloud gate fails on main, so every commit lands red, and
gh run list hides it: it lists only Actions workflows, and the
failure is an app check run.

Reliability rests on one bug, in test_file_service.py, which wrapped
an assertion in an except Exception re-raised through pytest.fail.
Removing it takes the rating from D to A.

Two pieces of debt ride along. The SDK callback id now comes from
crypto.getRandomValues, since it guards an endpoint with no auth. And
core/tasks gets the __init__.py that lets pylint see it, with the
debt that exposes, which is why #1533 fails lint-back.

src/backend/core/tasks/_task.py

@@ -1,4 +1,11 @@
+"""
+Celery task decorator that degrades to a synchronous call when Celery is off.
+"""
+
+# The Celery app is imported lazily so that importing this module does not pull
+# in Celery when CELERY_ENABLED is false.
 # ruff: noqa: PLC0415
+# pylint: disable=import-outside-toplevel
 
 from django.conf import settings